1. Technical Field
The present invention is related to a counter circuit and, in particular, to a high-speed hybrid structure counter having synchronous timing and asynchronous counter cells.
2. Discussion of Related Art
Multi-bit counter circuits are needed in many circuit applications, for example in phase-locked loop frequency dividers, and spread spectrum control dividers. In typical applications, counter circuits are at least 16-bits in size, and work at a frequency up to about 800 MHz. Counter circuits used in these typical applications may be either asynchronous or synchronous in their operation. In a synchronous counter circuit, there is a common source of clock pulses driving all of the components of the circuit. In an asynchronous counter circuit, components of the circuit are not driven by a common clock signal, and each component may be driven by a separate clock signal.
An asynchronous frequency divider has a relatively simple structure, and therefore occupies the least area, allowing for increased integration. Asynchronous frequency dividers, however, are delay dependent and are difficult to operate at high frequencies. A synchronous frequency divider, on the other hand, is more stable at nominal and high frequencies, but it has a relatively complicated circuit design, resulting in a larger footprint and increased power consumption, which makes synchronous designs more difficult to integrate. Moreover, synchronous frequency dividers often have a complex control logic, and a long delay path, which impedes the divider from operating. Typically, a delay cell or delay circuit is incorporated into synchronous frequency dividers to compensate for variable delay paths to improve operation at higher frequencies. However, because delay cells can have a large variance, and are often temperature and voltage sensitive, the use of these delay cells often have a negative impact on circuit performance.
Therefore, there is a need for more robust counter circuits capable of operating at higher frequencies while having simpler circuit design and smaller footprints.